Polymerization of monomers in microemulsions is known to those skilled in the art. They are stable, transparent water-in-oil systems that are stabilized by surfactants. Water soluble polymers such a polyacrylamide are effective in papermaking to improve drainage, formation and retention. Fast drainage and greater retention of fines contribute to lower cost. In addition, they are useful in the flocculation of suspended solids, such as sewage sludge and in the thickening of cellulosic paper pulp suspensions. The increasing cost of materials has made it highly desirable to produce flocculating agents which produce higher separation at lower dose levels. Finally, they are used in enhanced oil recovery processes as drive fluids to push through underground oil reservoirs.
Polymerization of monomers in emulsions is well known to those skilled in the art. Polymers produced by these techniques have found widespread industrial application. Further, some of the techniques described in the literature disclose a variety of multi-stage addition techniques for use in emulsion polymerization methods. Typically, the prior art emulsion disclosures teach adding the second stage as an emulsion and have high aqueous content or no oil.
The Rohm and Haas product literature entitled "Emulsion Polymerization of Acrylic Monomers" pp. 7, 14-18, teaches multi-stage emulsion polymerization of ethyl acrylate to achieve higher solids (43-46%) and to control heat. However, the reference discloses polymerization in an aqueous emulsion with the later steps comprising further addition of the aqueous emulsion.
Naidus, "Emulsion Polymers for Paints," Industrial and Engineering Chemistry, v. 45, n. 4 (1953), discusses adding monomer, or monomer in an aqueous emulsion, continuously during the polymerization to provide a homogeneous composition. The author teaches that the monomer addition technique gives emulsions of smaller aqueous droplet size because of a larger emulsifier to monomer ratio; and that the monomer emulsion addition technique is more stable with less coagulum since adequate emulsification is not dependent upon the agitation.
Taft, U.S. Pat. No. 3,297,621, teaches a two step emulsion polymerization process to control heat wherein the first step comprises adding nonemulsified monomer to a reactor containing catalyst and an emulsifying solution and the second step comprises adding an aqueous emulsion of monomer to the reactor.
Morgan, "Multifeed Emulsion Polymers," J Appl . Polymer Sci., v. 27, 2033-42 (1982), teaches a two stage emulsion to form core/shell concept macroemulsions. The author teaches a first step of continuously adding monomer to surfactant and water; and a second step of adding monomer as a water-in-oil emulsion to minimize destabilization of the seed polymer. The changing of the feed from monomer to water-in-oil emulsion may cause HLB problems.
Robinson et al., U.S. Pat. No. 5,110,864, teach a cationic monomer delayed addition process to produce a polymer having improved retention properties vis-a-vis polymers produced in a comparable one-step process. The disclosed monomers are cationic although acrylamide and acrylic acid are mentioned as comonomers. The patentees teach adding a portion of the monomer containing aqueous phase to the oil phase, emulsifying, adding the remaining portion of the aqueous phase without polymerization, and then polymerizing.
Also known in the art is the use of inverse microemulsion polymerization techniques. The mechanism and reaction kinetics in inverse microemulsions are different than those observed in inverse emulsions. The formation of microemulsions is considerably more complex than the formation of emulsions. Inverse emulsions typically contain 1-10 micron droplets and size grows continuously. The microemulsion polymerization techniques of the prior art are either one-step processes or add the second portion as an emulsion and require a relatively high surfactant and oil content and in some cases disclose a transparent monomer microemulsion.
Candau et al., U.S. Pat. No. 4,521,317, teach a process for polymerizing a water soluble monomer in a water-in-oil inverse microemulsion. The patentees teach that the monomer emulsion is a transparent microlatice and that the aqueous phase comprises 1-50 percent by weight of the total.
Durand et al., U.S. Pat. No. 4,681,912, teach a process to manufacture inverse microlatices of water soluble copolymers by admixing an aqueous phase containing water-soluble monomer and an oil phase with non-ionic surfactant(s) having an HLB range of 8-11 to form a transparent monomer microemulsion and polymerizing. The patentees teach determining the minimum surfactant concentration according to the formula: y=5.8x.sup.2 -110x+534 where x=HLB value and y =surfactant concentration.
Holtzscherer et al., "Application of the Cohesive Energy Ratio Concept (CER) to the Formation of Polymerizable Microemulsions," Colloids and Surfaces, 29 (1988), discuss the use of the cohesive energy concept to determine the most efficient use of surfactants in microemulsions. The minimum surfactant content found was 10.8 percent and an optimum HLB of 8.68. Monomer content was 14-22.5 weight percent.
Dauplaise et al., U.S. Pat. No. 4,954,538, teach crosslinked glyoxylated (meth)acrylamides prepared using inverse microemulsion techniques which are disclosed to be useful as wet- and dry- strength agents in paper production.
Honig et al., EP 0 462 365, discuss the use of ionic organic microemulsions to provide improved products useful in drainage and retention in papermaking processes.
Holtzscherer et al., "Modification of Polyacrylamide Microlatices By Using A Seeding Procedure," and Holtzscherer et al., K. L. Mittal and P. Bothorel, eds. Surfactants in Solution, in press, teach a seeding procedure applied to inverse acrylamide microlatices to increase polymer content. Higher solid contents are desirable in most industrial applications. However, the acrylamide is precipitated after the first step. The polymer content is 2.02-4.38 weight percent after the first step and 8.22-10.29 weight percent at final. In addition, the oil phase is 88-92 weight percent.
While the prior art microemulsion processes have provided improvements in the performance of the polymeric products, there still exists a need in the art for further improvement. The amounts of oil and emulsifier employed in the prior art processes are relatively high, thereby making the products more costly.
It is therefore an object of the present invention to produce water-in-oil microemulsions with smaller aqueous phase droplet size using equivalent surfactant and oil content or to produce water-in-oil microemulsions with equivalent polymer phase droplet sizes using significantly less surfactant. The multistep emulsion products also have superior performance as retention aids, in sludge dewatering and as oil recovery drive fluids compared to polymer products of a one-step microemulsion.